This invention relates generally to the attachment of solder balls to semiconductor packages such as ball grid arrays. More particularly, it relates to methods and apparatus for the accurate placement and alignment of solder balls by positioning them in the hole array of a stencil prior to attachment to a substrate.
As technology advances, the complexity, functionality and speed of integrated circuit (IC) chips is steadily increasing. These increasingly complex, high speed IC chips often require correspondingly increasing numbers of electrical interconnections due to their increased functionality. Consequently, high density interconnect package assemblies and the trend toward miniaturization have been principal objectives of semiconductor manufacturers. To this end, there have been semiconductor packages that have been developed with these objectives in mind. One such example is a ball grid array (BGA), which is diagrammatically illustrated in FIGS. 1a and 1b. As seen therein, an integrated circuit die 2 is affixed and bonded to a substrate 4 which is then covered with a cap 7 to provide protection from the outside environment. FIG. 1b is a bottom view of the package of FIG. 1a showing the array of solder balls. The solder balls permit a relatively high interconnect density to be incorporated in a package having a relatively small footprint. The solder balls 8 are formed in a dense pattern of a grid array 8 along the bottom surface of substrate 4 and are arranged to be received on a circuit board having corresponding electrical contacts, for example.
In manufacturing semiconductor packages that utilize arrays of solder balls such as ball grid arrays (BGAs), one of the major difficulties is accurately aligning and attaching the solder balls to the bottom of the substrate in a way that is efficient and avoids costly rework for defective packages. Various solder ball attachment machines have been used in the past which tend to be very expensive thereby limiting their widespread use. Some of these machines incorporate a high level of automation and are very complex. These may also utilize vacuum assist to pick and place whole arrays of solder balls simultaneously in an effort to meet high production requirements. The cost of these machines are typically in the range of $100,000 to $400,000 and the price of some highly automated machines run even higher if they include an optical sensing system that include cameras to detect missing balls prior to attachment.
These solder ball attachment machines often operate by first applying a controlled thickness of flux to a molded BGA substrate strip typically by means of a "doctor blade." A vacuum pickup head is then lowered into a vibrational pot containing a reservoir of preformed solder balls where the head is designed to simultaneously pickup a complete array of solder balls. A vacuum is present to draw the solder balls into an array of holes in the head, the holes are sized so that only one ball fits in each hole which are held in place by the vacuum until released on the substrate.
A camera system is used to detect missing balls in the array or locations where the head happened to miss picking up balls. If a defect is detected, the balls are promptly released and the pickup is reinitiated. If all of the balls are present, the pickup head moves to a fluxing station where the solder balls are be dipped into a controlled flux layer. The pickup head then moves to the substrate strip and transfers the solder balls onto the individual landing pads on each individual unit. The operation is continued in a fixed progression for subsequent strips until all solder balls are attached to the strip. The strip is subsequently moved to a furnace for solder reflow.
Although the described system works fairly well, there have some reliability problems in the past with their use. Most notably, the vacuum pickup head will shut down when flux is inadvertently drawn into the nozzle during the dipping operation at the flux station. Lost production due to machine down time is very costly and must be avoided. Cost is also a major factor that precludes widespread use of a fully automated system which can run upwards to a half a million dollars. What is needed is a relatively inexpensive solder ball attachment system that is accurate, reliable, and able to be outfitted for mass production.